How to Design a Switch Debounce Circuit

What it does

This circuit converts a noisy switch input signal to a clean, debounced logic signal.

How it works

Assume the circuit has reached steady state, with C1 fully charged and TP1 at 0 volts. When S1 closes, its contacts momentarily bounce (before fully closing) causing paths A and B to alternatively charge and discharge capacitor C1 during the bounce period. Since R2 is much larger than R1, path A takes longer to charge C1 than it takes path B to discharge C1, essentially clamping C1 to a near-zero voltage (of the R1-R2 voltage divider) despite the switch bounce; TP1 becomes VCC volts. When S1 is re-opened, the voltage across C1 slowly increases as it charges through R2, changing TP1 back to 0 volts. The Schmitt trigger inverter converts the slowly changing R-C signals into crisp logic outputs for use by microcontrollers or other digital devices.